Filters having hollow fiber membranes are used i.e. in the area of dialysis for a wide variety of purposes. Such filters are used, for example, in haemodialysis, in which blood is directed into and through the inside of the semi-permeable walls of the hollow fibers while dialysis fluid is directed around the outside of the hollow fibers. Various convection and diffusion processes thereby take place through the walls of the hollow fibers, which processes serve to purify the blood and to remove excess fluid. Additionally, due to this, the electrolyte concentration in the blood can be conditioned, and buffers such as bicarbonate or acetate can be added to the blood.
Filters of this type are also employed in so-called haemofiltration, in which a substitution fluid is added to the blood. According to this process, the blood is directed through the inside of the hollow fibers, although in this case no dialysis fluid is passed around the outside of the fibers. Here, in the filter, excess fluids, in particular water as well as waste products, are removed from the blood only by means of a pressure difference across the membrane, i.e. the semi-permeable walls of the hollow fibers. The substitution fluid can be added either before or after the filter.
The above-mentioned filters can equally be used as so-called ultrafilters for the production of the substitution fluid. In this case, dialysis fluid can for example be directed through the inside of the hollow fibers and can, by means of a pressure difference, be filtered through the membrane or semi-permeable walls, whereby the dialysis fluid can be sterile-filtered as a result of the removal of endotoxins, bacteria and other contaminants.
Further uses for the filters mentioned include haemodiafiltration; a combination of haemodialysis and haemofiltration, as well as plasmapheresis, in which the aqueous blood-plasma is filtered out of the blood and returned to the blood again after treatment. Such filters are also used in reverse osmosis.
The aforementioned filters are usually so constructed that the hollow fibers are arranged as a loose bundle collected together within a tubular housing. The housing is provided at each end with an end-cap, whereby the hollow fiber bundle is arranged between the ends of the housing so that the end-caps cover the ends of the hollow fiber bundle. A support ring is provided around each end of the hollow fiber bundle, while the ends of the hollow fibers are embedded within a potting compound at the ends of the housing.
The hollow fibers collected together in the hollow fiber bundle have ends which open into a hollow space located between the end cap and the end of the hollow fiber bundle. It is therefore possible, with the appropriate arrangement of inlets and outlets, to provide, in a known manner, various forms of filter, such as the previously mentioned haemodialysis filters, haemofilters, haemodiafilters, ultrafilters etc.
Examples of the previously mentioned filters are disclosed in EP-0 305 687, EP-0 355 325 and EP-0 525 317.
Essentially, all the previously mentioned filters have certain features in common, namely that a first fluid is directed into and through the inside of the semi-permeable hollow fibers while a second fluid is present on the outside of the hollow fibers. This second fluid can either flow through the housing, past and around the hollow fibers, via appropriately located inlet and outlet means; or it can for example be removed from the first fluid and directed out of the housing via a suitable outlet, for example by means of a pressure differential across the hollow fiber membrane.
A disadvantage of the previously mentioned filters can be seen in that during the usual heat-sterilization process, for example using steam, shrinkage of the hollow fibers occurs, in particular with synthetic hollow fibers. These mainly axial shrinkages of the hollow fibers can lead to damage or in worse cases to leakage, especially when the extent of the shrinkage cannot be compensated for by the elasticity of the hollow fibers.
It is clear, especially during the above-mentioned uses of these filters, that any leakage of the hollow fiber membrane should be avoided in all circumstances. A leakage in just a single fiber could lead to a serious risk for a patient.